This invention relates to an ink composition, especially for inkjet printing means, which comprises an aqueous liquid vehicle in which solid pigment particles are dispersed. Furthermore, the subject matter of the invention is an ink printing process matched to this ink composition and an ink printing means for executing this ink printing process.
Pigment inks are essentially suspensions in which the actual coloring agent is present as a solid in the form of pigment particles which are dispersed in a liquid vehicle, usually water-based. Pigments can be organic or inorganic coloring agents on the condition that they are insoluble in the liquid vehicle. By the choice of these pigments which adhere to the printed surface after the ink dries, printouts with high color saturation and permanency can be produced which cannot be achieved with water-soluble, dye-based inks. By adding polymers which crosslink after printing, water-resistant printouts can be produced.
One basic requirement for pigment ink compositions is that the dispersion is stable over the long term, as much as possible even at elevated temperatures. This is of special importance in storage tanks or ink cartridges for inkjet printers and plotters in which the ink properties must remain as uniform as possible even when stored for a longer time and when the temperature and climate fluctuates. To ensure that the pigment particles are not able to precipitate out of the liquid vehicle and lead to malfunctions by formation of deposits and residues in the ink feed lines and printing nozzles, surface-active substances, for example, surfactants, are added as dispersion agents and settling inhibitors. By adding these additives which have a molecular structure with a hydrophilic and hydrophobic end, wetting of the suspended pigment particles is increased. In this way a certain stabilization of the dispersion can be achieved. Exact determination and matching of the additive combination to each individual pigment used are necessary since matching to interfacial properties which are different in each case between the pigment substance and liquid vehicle is necessary. This results on the one hand in considerable development cost and leads on the other hand to the fact when using different pigments divergent properties of inks can hardly be avoided.
Another problem arises in water-resistant pigment inks in that they must additionally contain resins or polymers which provide for water-resistant fixing of pigment particles on the printed surface. Because these substances on the one hand during printing are to permanently set, for example by water-insoluble crosslinking during drying, on the other hand the fine printing nozzles and ink supply channels however should not be adversely affected in any case, a conflict of objectives arises which can only be resolved by compromises, for example the addition of substances for delay of drying and for regular mechanical cleaning of the printing nozzles to prevent deposits. This leads both to deterioration in the drying behavior and in water resistance and also to possible adverse effects on the printing system by the unwanted fixing of ink residues there.